1. Field of the Invention
The present invention relates generally to surface cleaning and, more particularly, to a method and apparatus for megasonic cleaning of a semiconductor substrate following fabrication processes.
2. Description of the Related Art
Megasonic cleaning is widely used in semiconductor manufacturing operations and can be employed in a batch cleaning process or a single wafer cleaning process. For a batch cleaning process, the vibrations of a megasonic transducer creates sonic pressure waves in the liquid of the cleaning tank which contains a batch of semiconductor substrates. A single wafer megasonic cleaning process uses a relatively small transducer above a rotating wafer, wherein the transducer is scanned across the wafer, or in the case of full immersion a single wafer tank system is used. In each case, the main particle removal mechanisms by megasonic cleaning are due to cavitation and acoustic streaming. Cavitation is the rapid forming and collapsing of microscopic bubbles in a liquid medium under the action of sonic agitation. Upon collapse, the bubbles release energy which assists in particle removal through breaking the various adhesion forces which adhere the particle to the substrate. Acoustic streaming is the fluid motion induced by the velocity gradient from propagation of the acoustic wave through the liquid medium under megasonic vibration.
FIG. 1A is a schematic diagram of a batch megasonic cleaning system. Tank 100 is filled with a cleaning solution. Wafer holder 102 includes a batch of wafers to be cleaned. Transducer 104 creates pressure waves through sonic energy with frequencies near 1 Megahertz (MHz). These pressure waves, in concert with the appropriate chemistry to control particle re-adhesion, provide the cleaning action. Because of the long cleaning time required for batch cleaning systems, as well as chemical usage, efforts have been focused on single wafer cleaning systems in order to decrease chemical usage, increase wafer-to-wafer control, and decrease defects in accordance with the International Technology Roadmap for Semiconductors (ITRS) requirements. Batch systems suffer from another disadvantage in that the delivery of megasonic energy to the multiple wafers in the tank is non-uniform and can result in ‘hot spots’ due to constructive interference, or ‘cold spots’ due to destructive interference, each being caused by reflection of the megasonic waves from both the multiple wafers and from the megasonic tank. Therefore, a higher megasonic energy must be applied in order to clean all regions of the wafers in wafer holder 102.
FIG. 1B is a schematic diagram of a single wafer cleaning tank. Here, tank 106 is filled with a cleaning solution. Wafer 110, which may be supported by a carrier, is submerged in the cleaning solution of tank 106. Piezo-electric crystals 115a–d are bonded to resonator 112 and provide the acoustic energy to assist in cleaning wafer 110. As it is easier to work with smaller piezo-electric crystals, a number of smaller crystals are typically bonded together with a resonator. However, this configuration causes gaps 114a–c between the respective crystals. Gaps 114a–c result in regions 116a–c not being supplied with a comparable level of acoustic energy as compared with regions 117a–d, due to the collimated nature of the acoustic energy supplied by piezo electric crystals 115a–d. Consequently, certain regions of wafer 110 do not see a uniform acoustic energy, resulting in uneven cleaning.
In view of the foregoing, there is a need for a method and apparatus to provide a single wafer megasonic cleaning configuration that is capable of distributing acoustic energy evenly over the surface of a wafer irrespective of the piezo crystal configuration in order to provide uniform cleaning across a surface of the wafer.